The objective of this research is to measure and simulate the function of biarticular muscles during human walking. The clinical treatment of locomotor impairments often includes targeted surgical and rehabilitative interventions performed on biarticular muscles. However, it can be extremely challenging to predict a priori how different treatments will alter an individual's gait. Computational models of the musculoskeletal system provide a systematic way of predicting how muscles actuate movement. It has previously been shown that model predictions are often non-intuitive, and sometimes inconsistent with assumptions that underlie current treatment strategies. However, the accuracy of the model predictions has not been established, which limits the impact of the models on treatment. The investigators in this study use electrical stimulation experiments to directly measure how two biarticular muscles, the rectus femoris and hamstrings, biomechanically function during walking. Abnormal activation of these muscles is often implicated as a cause of gait abnormalities that are characterized by diminished knee flexion during the swing phase of walking, and/or excessive knee flexion during the stance phase. In the experiments, subjects walk at a constant speed on an instrumented, split-belt treadmill. At select phases of a random gait cycle, electrical muscle stimulation is then used to alter the normal activation of the rectus femoris or hamstrings. The resulting perturbations to walking kinematics are recorded using a motion analysis system. Comparison of un-perturbed and perturbed walking provides a basis of assessing the movement induced by the individual muscles. The data are used to test the hypotheses that over-activation of the rectus femoris during stance induces a more extended limb during swing, while over-activation of the hamstrings during swing induces a more flexed limb during stance. Measurements are compared to computational model predictions, so as to rigorously evaluate the accuracy of assumptions regarding musculoskeletal geometry and muscle force transmission paths. The anticipated outcomes of this study are an enhanced understanding of biarticular muscle function during walking, and improved confidence in the use of computational models to evaluate surgical and rehabilitative treatments of locomotor impairments. PUBLIC HEALTH RELEVANCE: Locomotion impairments are common among individuals with neurological disorders such as cerebral palsy. Abnormal movement patterns can greatly increase the metabolic cost of walking and contribute to long-term joint degeneration and physical disability. For this reason, surgical and/or rehabilitative treatments are often used to try to correct abnormal gait patterns. However, it can be challenging to predict how different treatment options will affect a patient's gait. This study uses experimental and computational techniques to assess how muscles function normally during walking, so as to contribute to a scientific basis for establishing effective interventions.